1. Field of the Invention
This invention relates to a transmitting method and apparatus for a compressed picture signal. This invention can be applied to a recording apparatus for recording a moving picture on a disc-like recording medium rotated with constant linear velocity.
2. Description of the Prior Art
As a disc-shaped recording medium that rotates at constant line velocity, a digital audio disc (also called a compact disc and abbreviated as "the CD") is wellknown. In a CD, in order to take advantage of digital recording and have greater capacity, a CLV (constant line velocity) system has been adopted. Recording a picture signal as well as a digital audio signal onto the CD has been considered. However, the information amount of the picture signal is much greater than the audio signal, so an effective data compression technique is needed for both still pictures and moving pictures. For compression of picture data, highly efficient encoding is employed.
Highly efficient encoding can be classified into intraframe encoding (one dimension and two dimensions) and interframe encoding. The intraframe encoding performs processing on a scanning line (one dimension), or intrafield or intraframe processing (two dimensions), while the interframe encoding performs three-dimensional interframe processing. The intraframe encoding provides a low compression rate, but it can provide a reproduced picture of high quality. On the other hand, the interframe encoding provides a high compression rate, but there are problems that the picture quality of a reproduced picture is inferior to that in the intraframe encoding. Additionally, in the interframe encoding, a propagation error is generated.
Such highly efficient encoding is employed in a communication system such as a video conference or a video telephone, which requires the high compression rate. To prevent the generation of a propagation error, the first picture is subjected to intraframe processing and the remaining ones are subjected to interframe processing. This is called adaptive selection of the intraframe processing and the interframe processing in these communication systems. However, in the case of the CD, data can be obtained from the CD intermittently at the time of a special reproduction such as random access, search or reverse. For this reason, when a frame difference is encoded with the interframe processing as mentioned above, there is a problem that a reproduced picture cannot be provided.
To solve this problem, a system where one picture is completely subjected to the intraframe processing periodically and where the remaining pictures are subjected to the efficient interframe processing is proposed. With this system, a reproduced picture can be provided intermittently by reproducing intraframe-encoded picture data from the CD at the time of the above-mentioned special reproduction. Consequently, the special reproducing operation is possible.
As one example, one frame is subjected to the intraframe encoding at a period which is equal to six continuous frames in time, and the remaining five frames are subjected to the interframe processing. As a result, as shown in FIG. 1, encoded data F1 (indicated by hatching area) corresponding to picture data of the first frame, which is generated by the intraframe encoding, is positioned in a two-frame period, for example, of the six frames. Encoded data is generated in the remaining four-frame period so that encoded data F2 to F6 correspond to picture data of the second frame to the sixth frame.
FIG. 2 shows a part of the innermost circumference side of a spiral track of the CD. Recording data is shown in FIG. 3A in the state where the track is developed linearly by the x--x' line in the radial direction of the disc. As described above, the period of the intraframe encoding is set at 6 frames, and data generated in the intraframe encoding (indicated by each hatched area) is inserted over a two-frame period. The intraframe-encoded data is recorded every six frames from data F1 corresponding to the first frame in a part of the disc shown in FIG. 2.
During the special reproduction operation, for example, the pick-up feed speed in the diameter direction of the disc is performed at high speed as compared with the normal reproduction operation and during the search operation, where a track jump takes place, only data subjected to the intraframe encoding is reproduced sequentially as shown at the arrows of FIG. 3A. Reproduction data corresponding to FIG. 3A is shown in FIG. 3B. In FIG. 3B, each hatched area represents the cue time (i.e., the total time required for a track jump of the pick-up and for reaching the next reproduction of intraframe encoded data). Further, in FIG. 3B, the display operation of a reproduced picture corresponding to reproduced data is shown. Clearly, a reproduced picture P1 provided by the intraframe decoding of the reproduction data F1 is repeatedly displayed on a monitoring device until a reproduced picture P7 of the next seventh frame is obtained. As will be understood from FIG. 3B, a postsix frame picture is provided every two-frame or three-frame period.
Since the efficiency of the intraframe processing is poor in the encoding system which uses the above-mentioned special reproduction, data generated in the intraframe processing exceeds one frame requiring a two-frame period (the above-stated example) or a much longer period. Therefore, at the time of search operation, the same picture is repeatedly reproduced until the reproduction of data subjected to the next intraframe processing is completed. For this reason, there is a disadvantage that the change of a reproduced picture at the time of reproduction is slow.
To solve this disadvantage, a proposal that the encoding of a frame to be subjected to the intraframe processing is done by dividing the frame into two stages has been made. Specifically, in the first stage, after an original picture is subjected to band restriction, one half of the data is thinned out in the longitudinal and lateral directions to reduce the data amount to one fourth. By processing this data, the generated data amount is kept within one frame. At the second stage, the picture provided in the first stage is returned to its original size by interpolation, and after obtaining a difference between the picture and the original picture, the difference is encoded.
According to this step-by-step encoding system, a picture of low resolution provided in the first stage is reproduced at the time of search. A reproduced picture which varies smoothly every frame time can consequently be obtained. However, there are problems that the circuit size becomes large since circuits for the thinning-out and the interpolation are needed because of the two stages of decoding and that the efficiency is deteriorated.